Salmonella spp. is a highly contagious gram-negative intestinal pathogen with strong endotoxin and invasiveness, belongs to intracellular bacteria, and can attach small intestinal mucosal to cause diseases such as enteric fever, gastroenteritis and septicemia, even severe intestinal bleeding or perforation. For most serotypes of Salmonella, their median infective doses are between 105 and 108, but in epidemic outbreaks, the infective doses are generally less than 103 bacteria, sometimes even less than 100 bacteria. In Asian countries, especially in China, the proportion of intestinal diseases caused by Salmonella paratyphi A is increasing, and some studies find that there are 150 cases of Salmonella paratyphi A infection per 100 000 people per year.
At present, the main way to treat typhoid and paratyphoid is antibiotics, but with the emergence of drug-resistance, especially the emergence of multiple drug-resistant strains, conventional antibiotic treatment encounters a huge challenge, and immunization of relevant vaccines is an effective means of prevention. At present, the progresses of research and development for oral attenuated live vaccine against Salmonella typhimurium, Vi capsular polysaccharide vaccine and Vi polysaccharide-protein conjugate vaccine are rapid, and there are a variety of products listed, but these vaccines are not able to generate cross immunoprotection against Salmonella paratyphi A. Currently, there is not a vaccine against Salmonella paratyphi A that has been approved for listing.